During its transmission in a radio communication network, a signal is affected by disturbances of the propagation channel on which it is emitted. These disturbances have the effect of modifying the received signal compared with the emitted signal. These modifications to the received signal degrade the signal to noise ratio.
For example, one conventional manner of limiting the influence of disturbances of the propagation channel on a transmitted signal, is to use a signal processing method intended to transform the received signal into a processed signal such that the processed signal is closer to the emitted signal than the received signal. Thus, as the performance of the signal processing method improves, the processed signal becomes closer to the emitted signal and consequently the signal to noise ratio becomes higher. This provides a method of improving the performances of these reception systems by using a high performance signal processing method. Note that in the context of this document, the term “noise” refers to both decorrelated noise and correlated noise, in other words also interference noise.
Conventionally, a signal reception system may be adapted to receive signals on P antennas, where P is a number greater than or equal to 1, and to generate a processed signal by applying one of these signal processing methods to the signal(s) received by the corresponding antennas.
Regardless of the number P of antennas that receive the emitted signal, the performance level of such a reception system is dependent on the processing method applied to the signal(s) received on the antenna(s) to generate the processed signal.
Some of these signal processing methods are used firstly to estimate a value of transmitted information, and also to estimate a likelihood value of this transmitted information. This likelihood value represents an image of the probability that the estimate made is correct. A likelihood with a high absolute value indicates that the estimate made is probably correct, and a likelihood for which the absolute value is low indicates that the estimate made only has a low probability, for example of the order of 55 to 60%, a likelihood with a zero value indicates that there is no way of deciding whether the value of the information transmitted is 0 or 1. The sign of a likelihood value indicates the value (0 or 1) of a corresponding binary information estimate. Conventionally, a likelihood value (V) satisfies the following equation:
      V    ⁡          (      X      )        =      Log    ⁡          (                        Probability          ⁡                      (                          X              =                              0                /                Y                                      )                                    Probability          ⁡                      (                          X              =                              1                /                Y                                      )                              )      
where Log(z) represents the Napierian logarithm of z;
where X is the emitted information for which an estimate is required;
where Y is the received signal corresponding to the transmitted information that is to be estimated.
Such an expression means that a likelihood value of information X is equal to the Napierian logarithm of the ratio of the probability that the information X was emitted with the value 0 knowing that the corresponding received signal is Y, to the probability that the information X was emitted with the value 1 knowing that the corresponding received signal is Y. In this context, the probabilities are usually calculated assuming that noise present during the transmission and interference globally correspond to a Gaussian distribution noise and the variance of this Gaussian distribution corresponds to the average noise power.
An estimate of the average noise power presents on a given propagation channel may be very useful to the receiver, particularly to estimate a likelihood value of transmitted information.
For example, the likelihood of transmitted information (also called “soft bit”) is conventionally used at the input to channel decoding processing on reception, when a channel coding processing is done at the emission. Use of a likelihood value as a basis rather than only an information value (also called hard bit) indicated by the sign of the likelihood of the information, provides a means of significantly improving the channel decoding performances and therefore the system performances in terms of resistance to errors, for example due to propagation on the transmission medium.
For example, one of these signal processing methods is called the “antenna diversity method”. One antenna diversity method consists of estimating the initially transmitted signal from the received signal based on a propagation channel estimate, for each antenna, and then to summating the signals thus estimated on each antenna to generate a processed signal, then called a “composite signal” in the framework of multi-antenna systems. A value of an average noise power can be estimated for such an “antenna diversity method”.
Therefore this type of signal reception system takes account of information about the reception noise of the signal, for example so as to calculate information likelihood values as precisely as possible. In such a context, as the information about noise becomes more accurate and more correct, the performance of the multi-carrier signal reception system can improve.
This invention is designed to improve information about noise supplied in this type of signal reception system, and thus improve the performances of such a system.